>I understand that the default for SunOS 4.1.x fo udp checksumming is off

>I've found from the sun-managers archives that I can turn this on using adb
>with
>udp_cksum?W1

(And reboot, thjis changing only /vmunix, not the in kernel value.
or also do adb -k -w /vmunix /dev/mem and udp_cksum/W1)

>but - what does this give me? Will UDP packets that don't meet the checksum be
>ignored? What happens if that occurs half way through an NFS file operation -
>will NFS handle this correctly, and resend - if so how does it know, if UDP
>doesn't ACK packets?

Yes. The bug in SunOS 4 is that with udp_cksum not set, incoming checksums
are ignored. It is up to the protocol implemented on top of RPC
to do the retransmissions (NFS, DNS, ONC RPC will all retransmit after a time
out)